Leishmania potozoans are the causative agents of human leishmaniasis, which includes a spectrum of diseases ranging from self-healing skin ulcers to fatal visceral infections. Human leishmaniasis is caused by at least thirteen different species and subspecies of parasites of the genus Leishmania. Leishmaniasis has been reported from about eighty countries and probably some 400,000 new cases occur each year. Recently, the World Health Organization has reported 12 million people to be infected (ref. 1--a listing of the references appears at the end of the disclosure).
L. donovani causes visceral leishmaniasis, also known as kala-azar. L. brasiliensis causes mucotaneous leishmaniasis and L. major causes cutaneous leishmaniasis. Untreated visceral leishmaniasis is usually fatal and mucocutaneous leishmaniasis produces mutilation by destruction of the naso-oropharyngeal cavity and, in some cases, death.
In addition, a major health problem has been created in areas of high infection when blood is collected for transfusion purposes. Since blood is a carrier of the parasites, blood from an infected individual may be unknowingly transferred to a healthy individual.
The Leishmania protozoans exist as extracellular flagellated promastigotes in the alimentary tract of the sandfly in the free-living state, and are transmitted to the mammalian host through the bite of the insect vector. Once introduced, the promastigotes are taken up by macrophages, rapidly differentiate into non-flagellated amastigotes and start to multiply within the phagolysosomal compartment. As the infected cells rupture, amastigotes subsequently infect other macrophages giving rise to the various symptoms associated with leishmaniasis (refs. 1 and 2). In this manner, it is the amastigote form of the parasite which is responsible for the pathology in humans.
While in the midgut of the insect, newly transformed promastigotes, functionally avirulent, progressively acquire capacity for infection and migrate to the mouthparts (ref. 3). This process, termed the metacyclogenesis, which occurs only in promastigotes, is concurrent with the differential expression of major surface glycoconjugates which mediate the migration of promastigotes in the alimentary tract and prepare the organism for the serum environment (refs. 4 and 5). In comparison, the promastigote to amastigote cytodifferentiation is a profound morphological and physiological transformation. During the promastigote to amastigote differentiation, the parasite looses its flagellum, rounds-up, changes its glycoconjugate coat (refs. 6, 7 and 8) and expresses a set of metabolic enzymes optimally active at low pH. The survival of the parasite inside the macrophage phagolysosome is associated with its ability to down-regulate many effector and accessory functions of its host cell, including oxygen metabolite-mediated killing and the capacity of the macrophage to act as an efficient antigen presenting cell (reviewed in, for example, ref. 9).
Leishmaniasis is, therefore, a serious disease and various types of vaccines against the disease have been developed, including live parasites; frozen promastigotes from culture; sonicated promastigotes; gamma-irradiated live promastigotes; and formalin-killed promastigotes treated with glucan (reviewed in, for example, ref. 10). However, none of these approaches have provided satisfactory results.
The promastigote-amastigote differentiation is important to the establishment of infection. It would be desirable to identify genes and gene products that are differentially expressed when the amastigotes are present in macrophages.
Joshi, et al. describe L. donovani genes that are expressed at about two-fold higher in in vitro generated and maintained "amastigotes" compared to promastigotes (ref. 11).